Single stage piston compressor or multistage piston compressor for cooling of an electrical motor for a single stage piston compressor or for a multistage piston compressor

ABSTRACT

A multistage piston compressor and a method for cooling of an electrical motor for a multistage piston compressor. Conventional piston compressors produce a high part of heat dissipation. This influences negatively the energy balance of the piston compressor and requires a high expenditure in cooling agents for conserving and treating with care the piston compressor. It is therefore disclosed to furnish the always present volume changeable free chamber ( 19 ) at the piston compressor ( 1 ) as a pressure chamber and a suction chamber, and to connect this volume changeable free chamber ( 19 ) to the atmosphere through the inner chamber of the electrical motor ( 2 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The Invention relates to a single stage piston compressor or to a multistage piston compressor according to the preamble of claim 5.

[0003] 2. Brief Description of the Background of the Invention Including Prior Art

[0004] Such piston compressors are employed in particular in the vehicle industry. Such a piston compressor in a two-stage embodiment is described in detail for example in the German printed Patent document DE 197 15 291 A1. The German printed Patent document DE 197 15 291 A1 is herewith incorporated by reference as if fully set forth herein. This two-stage piston compressor comprises a compressor casing forming a cylindrical low-pressure chamber with a larger low-pressure piston and a cylindrical high-pressure chamber with a smaller high-pressure piston. Here, the low-pressure chamber and the high-pressure chamber are disposed on a common axis and the low-pressure piston and the high-pressure piston are formed a single piece with a common piston rod. The low-pressure chamber has an entrance with an inlet check valve and the high-pressure chamber has an outlet with an outlet check valve and the two pressure chambers are connected by a by-pass channel and by an overflow check valve. A crank pin of a crankshaft engages in rectangular alignment into the common piston rod of the low-pressure piston and of the high-pressure piston, wherein the crankshaft is driven by an electrical motor and wherein the crankshaft converts the rotary motion of the crankshaft into a linear motion at the common piston rod.

[0005] Furthermore a free chamber is formed in the compressor casing, wherein the free chamber is disposed in the region between the low-pressure piston and the high-pressure piston and wherein the size of the free chamber results essentially from the difference of the diameters as well as from the axial distance between the low-pressure piston and the high-pressure piston. This free chamber is connected through an equalizing opening to the environment.

[0006] Piston compressors of this kind are associated with the disadvantage that an essential part of the drive energy is converted into heat and thus negatively influences the efficiency of the technical unit. In this way the electric motor produces its own heat, wherein this heat then has to be led away or cooled again in an expensive way and for the protection of the insulations. This limits on the one hand the lifetime of the electrical motors and requires additional and expensive cooling steps at the electrical motor.

[0007] In a like fashion heat is generated at the piston compressor primarily by the renewed compression of the air in particular in the high-pressure chamber but also because of an internal friction of the air disposed in the free chamber between the two pressure chambers. The internal friction of this enclosed air volume in the free chamber depends on the construction of the piston compressor, but at any rate represents a power dissipation for which an additional drive power has to be furnished in a non-justifiable manner.

[0008] The this way generated warming up at the piston compressor is either compensated to an acceptable measure by a corresponding material selection or material dimensioning or the released heat energy is again cooled expensively. This all interferes with the efficiency of the technical unit and renders the technical unit expensive and cost intensive because of the increased cooling provisions.

SUMMARY OF THE INVENTION

[0009]1. Purposes of the Invention

[0010] Therefore, it is an object of the present Invention to develop a piston compressor of this kind and a method for cooling of an electrical drive for such a piston compressor, where the generation of thermal heat loss is decreased at the piston compressor and wherein the cooling of the electrical motor is simplified. These and other objects and advantages of the present invention will become evident from the description which follows.

[0011] 2. Brief Description of the Invention

[0012] In accordance with the present invention, there is furnished a single stage or multistage piston compressor comprising a piston compressor proper, an electrical motor and a mechanical drive. The mechanical drive converts the rotating motion of the electric motor into an oscillating linear motion and transfers the oscillating linear motion onto the piston compressor proper. A single stage piston compressor comprises a volume changeable pressure chamber with a pressure piston. A multistage piston compressor has at least one volume changeable low-pressure chamber with a low-pressure piston and at least one volume changeable high-pressure chamber with a high-pressure piston. The low-pressure chamber and the high-pressure chamber of the multistage piston compressor proper are connected by a bypass channel. The low-pressure piston and the high-pressure piston are formed as a single piece. A volume changeable free chamber is disposed between the low-pressure piston and the high-pressure piston. The volume changeable free chamber of the piston compressor is furnished as a pressure and suction chamber. The volume changeable free chamber is connected to the atmosphere through the inner chamber of the electric motor.

[0013] The volume changeable free chamber advantageously has a connection to the outer atmosphere through an admission check valve. The low-pressure chamber can have a suction chamber predisposed. The suction chamber can be connected on the one hand to the low-pressure chamber and on the other hand to the inner chamber of the electrical motor and to the outer atmosphere.

[0014] The inlet opening of the suction chamber or a third casing opening of the electrical motor can be disposed outside of the dirt and water endangered region for safeguarding the kind of protection of the electrical motor.

[0015] There is also furnished a method for cooling of an electrical motor (2) for a single stage or multistage piston compressor. The air suctioned and ejected by a volume changeable free chamber of the piston compressor is employed as a cooling agent for the electric motor. The volume changeable free chamber is disposed between two neighboring pressure pistons of different size.

[0016] The air moved by the piston compressor can alternatingly be suctioned in from the atmosphere through the inner chamber of the electric motor and be removed into the atmosphere.

[0017] The air moved by the piston compressor can be separately suctioned in from the atmosphere and removed into the atmosphere through the inner chamber of the electrical motor.

[0018] The air moved by the piston compressor can be led simultaneously into the low-pressure chamber of the compressor through the inner chamber of the electric motor.

[0019] The Invention eliminates the recited disadvantages of the state of the art.

[0020] It is a particular advantage of the present invention that the required cooling steps for the heated piston compressor can be maintained small, since the heating up of the piston compressor can be maintained within limits by removal of the together warmed air in the free chamber of the piston compressor. This simplifies the construction of the piston compressor and saves production costs.

[0021] Of course it is also advantageous that the electrical motor is cooled for safeguarding the lifetime of the electrical motor only with the air stream of the piston compressor. This saves otherwise required expensive cooling steps for the electrical motor and saves in turn again production costs.

[0022] It is a particular advantage that no additional drive energy is required for the required air motion for limiting the heating up in the piston compressor and for cooling the electrical motor. This affects positively the efficiency of the piston compressor.

[0023] The piston compressors according to the present Invention can be employed for all kinds of protection.

[0024] The novel features which are considered as characteristic for the invention are set forth in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0025] In the accompanying drawing, in which are shown several of the various possible embodiments of the present invention:

[0026]FIG. 1 is a view of a schematic presentation of an electrically driven two-stage piston compressor according to a first embodiment, and

[0027]FIG. 2 is a view of a second embodiment of the two-stage piston compressor.

[0028]FIG. 3 is a view of a schematic presentation of an electrically driven two-stage piston compressor according to a third embodiment similar to FIG. 1, however with a reversed flow direction of cooling air as compared to FIG. 1,

[0029]FIG. 4 is a view of a fourth embodiment of the two-stage piston compressor combining features of the second and third embodiment,

[0030]FIG. 5 is a view of the embodiment of FIG. 1 with additional modifications, and

[0031]FIG. 6 is a view of the embodiment of FIG. 2 with additional modifications.

DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENTS

[0032] The invention is explained in more detail in the following by way of two embodiment examples.

[0033] The first embodiment of a two-stage piston compressor comprises, according to FIG. 1, mainly the piston compressor proper and an electrical motor 2, wherein both the piston compressor 1 and the electrical motor 2 are connected through a mechanical drive 3 for converting the rotary input motion from the electrical motor 2 into an oscillating and linear output motion of the piston compressor 1.

[0034] The piston compressor 1 comprises a compressor casing 4 with a cylindrical and stepped, with respect to diameter, inner chamber 5, wherein the inner chamber 5 is closed toward the outside on the one hand with a low-pressure cover 6 and on the other hand with a high-pressure cover 7. The inner chamber 5 of the compressor casing 4 comprises a first cylindrical hollow section of a larger diameter and a second cylindrical hollow section of a smaller diameter adjoining the first cylindrical hollow section and coaxially aligned with the first cylindrical hollow section. The stepping in diameter of the inner chamber 5 furnishes on the one hand a low-pressure chamber 8 with a larger diameter, wherein a low-pressure piston 9 is guided with play in the low-pressure chamber 8, and on the other hand a high-pressure chamber 10 with a smaller diameter, wherein a high-pressure piston 11 is fitted with play into the high-pressure chamber 10. Here, the low-pressure chamber 8 and the high-pressure chamber 10 are disposed on a common axis. The low-pressure piston 9 and the high-pressure piston 11 are formed as a single piece and correspondingly have a joint piston rod 12.

[0035] The low pressure chamber 8 is formed between the low pressure piston 8 and the low pressure cover 6 and an outer part of the first cylindrical section of a larger diameter.

[0036] The high pressure chamber 10 is formed between the high pressure piston 11 and the high pressure cover 7 and an outer part of the second cylindrical section of a smaller diameter. A free chamber 19 is formed between the low pressure piston 8 and the high pressure piston 11 and an inner part of the first cylindrical section of a larger diameter and an inner part of the second cylindrical section of a smaller diameter.

[0037] The low-pressure chamber 8 and the high-pressure chamber 10 are connected in a particular way amongst each other and toward the outside. Here, the low-pressure chamber 8 has at least one entrance 13 with in each case an inlet check valve 14 disposed in the low pressure cover 6, wherein the inlet check valve 14 opens up in the direction of the low-pressure chamber 8, and the high-pressure chamber 10 has an outlet 15 with an outlet check valve 16 disposed in the high pressure cover 7, wherein the outlet check valve 16 closes in the direction of the high-pressure chamber 10. A common piston rod 12 connecting centrally the low pressure piston 9 to the high pressure piston 11. A bypass channel 17 is running through the common piston rod for connection the low pressure chamber 8 to the high pressure chamber 10. The common piston rod 12 and the low-pressure piston 9 and the high-pressure piston 11 are equipped with a bypass channel 17 for a necessary connection of the low-pressure chamber 8 and the high-pressure chamber 10, wherein the bypass channel 17 has an overflow check valve 18 opening in the direction of the high-pressure chamber 10 and disposed on the high-pressure piston in the high pressure chamber 10.

[0038] The free chamber 19 results with the construction of the inner chamber 5 stepped in diameter and with the construction of the low-pressure piston 9 and of the high-pressure piston 11 within the inner chamber 5 and between the low-pressure piston 9 and the high-pressure piston 11. The free chamber 19 is required for the freedom of motion of the low-pressure piston 11 and of the high-pressure piston 11 and for the placing of the required elements of the mechanical drive 3. The mechanical drive 3 is surrounded by an, essentially closed mechanical drive casing 30. This mechanical drive 3 is for example a crankshaft drive. Correspondingly, the common piston rod 12 is equipped with means for force transmission from the mechanical drive 3 to the common piston rod 12.

[0039] According to the present Invention this free chamber 19 is connected to the inner chamber of the mechanical drive 3 formed by the mechanical drive casing 30 through at least a first casing opening 20 disposed in a common wall 32 of the mechanical drive casing 30 and of the compressor casing 4 in the area of the inner part of the first cylindrical section and further to an inner chamber of the electrical motor 2 formed by a motor casing 31 through at least a second casing opening 21 disposed in a common wall 33 of the mechanical drive casing 30 and of the motor casing 31. One or several third casing openings 22 disposed in the wall of the motor casing 31 connect the inner chamber of the electrical motor 2 to the atmospheric surroundings. These third casing openings 22 are disposed as far as possible remote from the second casing openings 21 in order to obtain a flow region as large as possible within the motor 2 and surrounding the motor 2. In order to assure a high protection kind for the electrical motor 2, the third casing openings 22 are placed outside of the dirt and water endangered regions by suitable means.

[0040] The second embodiment of the two-stage piston compressor according to FIG. 2 in principle has the same construction as the first embodiment. In contrast to the first embodiment, the second embodiment is equipped with an additional suction chamber 23, wherein the suction chamber 23 is disposed in front of the entrance 13 to the low-pressure chamber 8 as seen in flow direction and wherein the suction chamber 23 has, on the one hand, a connection to the atmosphere through an inlet opening 24 and, on the other hand, is connected to the inner chamber of the mechanical drive 3 through connection channel 25. Thus, the suction chamber 23 is also connected to the free chamber 19 in the compressor casing 4, and to the inner chamber of the electrical motor 2 through the first casing opening 20 and through the second casing opening 21. In contrast, the inner chamber of the electrical motor 2 is hermetically and sealingly separated from the outer environment. The suction chamber 23 can of course also be constructed as an open suction region.

[0041] For assuring of the required high kind of protection for the electrical motor 2, the inlet opening 24 for the suction chamber 23 is again disposed outside of the dirt and water endangered region by suitable means.

[0042] An advantageous embodiment for the two embodiments according to FIGS. 1 and 2 comprises that the free chamber 19 is connected to the atmosphere through one or several suction bore holes 26 disposed in the inner part of the first cylindrical section of a larger diameter or the inner part of the second cylindrical section of a smaller diameter together with a fitted admission check valve 27 opening in the direction of the free chamber 19. Continuously fresh and cool air is fed in from the outside for cooling of the motor 2, and warmed up air is released into the open through the third casing openings 22 in the motor casing 31 of the motor 2 or through the inlet opening 24 of the suction chamber 23.

[0043] The mode of operation of a piston compressor is generally known and the needs to be reproduced the only in essential parts.

[0044] The rotary motion of the drive shaft of the electrical motor 2 is transformed into an oscillating linear motion with the aid of the mechanical drive 3 and is transferred to the common piston rod 12. Here, in equal fashion the low-pressure piston 9, and the high-pressure piston 11 move between two oppositely disposed return points and thereby form two pressure chambers alternatingly changing in volume, that is the low pressure chamber 8 and the high pressure chamber 10. This course of motion and the functions of the inlet check valve 14, of the overflow pressure limiting valve 18, and of the outlet check valve 16 take care that air is batch wise suctioned from the atmosphere into the low-pressure chamber 8, is transported through the by-pass channel 17 into the high-pressure chamber 10, and is compressed and ejected from the high-pressure chamber 10.

[0045] The course of motion of the low-pressure piston 9 and of the high-pressure piston 11 entails at the same time that free chamber 19 of the compressor casing 4 alternatingly increases and decreases in size, such that the air disposed in the free chamber 19 alternatingly is ejected into the free surroundings by way through the first casing bore holes 20 into the inner chamber of the mechanical drive 3 and through the second casing bore hole 21 into the inner chamber of the electrical motor 2 and from there through third casing bore hole 22, and wherein the air disposed in the free chamber 19 in the following is again suctioned in a counter move. The air in this region is the held in motion thereby, and is practically moved back and forth. In the case of the use of an admission check valve 27 in the free chamber 19, air is suctioned from the outside through this admission check valve 27 and is released into the free surroundings through the remotely disposed third casing opening 22. The air moved in this way or in that way serves here as a chilling and cooling agent for the electrical motor 2.

[0046] The required air in case of a free chamber 19 increasing in size in the compressor casing 4, according to the second embodiment of the present Invention, is suctioned through the inlet opening 24, through the suction chamber 23, and through the connection channel 25, and in case of the use of an admission check valve 27 also through this admission check valve 27, and the free chamber 19 is therewith filled. In case of a free chamber 19 decreasing in size, the air disposed in the free chamber 19 is pushed out and transported through the connection channel 25 initially into the suction chamber 23 as is indicated by the arrows shown in FIG. 2. Since the low-pressure chamber 8 increases at the same stroke and suctions air in, the air made ready out of the free chamber 19 passes immediately again into the low-pressure chamber 8. Additionally, air is sucked in through the inlet opening 24 for volume balancing the air requirements of the low-pressure chamber 8 and of the air made ready from the free chamber 19. The static pressure relations during the filling process of the suction chamber 23 are automatically controlled such that a pressure is set in the suction chamber 23 supporting the opening process of the inlet check valve 14. This behavior of the inlet check valve 14 is additionally supported by the dynamic pressure relationships, which are caused by the flow and vibration forces of the moved air. This improves the filling process of the low-pressure chamber 8 and thereby improves the degree of effectiveness of the piston compressor 1. The air moved in this way between the free chamber 19 and the suction chamber 23 of course also flows through the inner chamber of the electrical motor 2 and thereby serves again as the cooling agent for the electrical motor 2.

[0047] If the admission check valve 27 is reversed in its direction and replaced by an exhaust check valve 28 as shown in FIGS. 3 and 4, then the volume changeable free chamber 19 sucks in the air from the atmosphere through the third casing opening 22 in the motor casing 31, through the second casing opening 21 in the mechanical drive casing 30 and the first casing opening 20 in the piston compressor. The exhaust check valve 28 is closed during the suctioning in of air from the motor 2 and the mechanical drive 3 and through the first casing opening 20 into the free chamber 19. In case of a decrease of the volume of the free chamber 19 the air enclosed in the free chamber 19 is partly released into the atmosphere through the exhaust check valve 28 and is partly pressed back through the casing openings 20 and 21 toward the motor. Since however the suction stream suctioned in from the atmosphere is larger as the subdivided pressure air flow pressed back into the motor, there occurs a continuous air motion from the motor 2 toward the piston compressor 1.

[0048] According to FIGS. 1, 2, 5, and 6 an air stream is sucked in by the free chamber 19 from the atmosphere and delivered to the motor for cooling.

[0049] According to FIGS. 3, and 4 an air stream is sucked in by the free chamber 19 from the motor casing 31 and from the mechanical drive casing 30 for cooling the motor 2 and the mechanical drive 3 and is discharged through the exhaust check valve 28 into the atmosphere.

[0050] A better control of the air flow can be obtained in case two check valves are employed as shown in FIGS. 4, 5, and 6. In general, the free chamber 19 will be associated with an admission check valve 27, 29 and with an exhaust check valve 28, 34, and 35. A first one of the check valves 27, 28 controls air flow between free chamber 19 and the ambient atmosphere and a second one of the check valves 29, 34, 35 controls the air flow between free chamber 19 and mechanical drive 3 and motor 2. When the first check valve is of the admission type, then the associated second check valve will be of the exhaust type and vice versa.

[0051] During a compression motion of the common piston rod 12 the exhaust type check valve 28, 34, 35 will be open and the admission type check valve 27, 29 will be closed. During a return motion of the common piston rod 12 the exhaust type check valve 28, 34, 35 will be closed and the admission type check valve 27, 29 will be open. Thus always one check valve will be open alternatingly together with the changes of direction of motion of the common piston rod 12. Thus according to FIG. 4 there will be a pulsating air flow from the motor 2 to the free space 19 and during the pulse intervals there will be air exhausted from the free chamber 19 into the atmosphere. In contrast, according to FIGS. 5 and 6 there will be a pulsating air flow from the free space 19 to the motor 2 and during the pulse intervals there will be air admitted from the ambient atmosphere into the free chamber 19.

[0052] The first one of the check valves 29, 34 is preferably disposed in the common wall of free chamber 19 and mechanical drive casing 30 as shown in FIGS. 4 and 5. It is also possible but less desirable to place the first one of the check valves 35 into the common wall of the mechanical drive casing 30 and the motor casing 31 as shown in FIG. 6 or into the wall of the motor casing 31 since the transported air volume will be relatively smaller in these cases.

[0053] It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of compression system configurations and cooling procedures differing from the types described above.

[0054] While the invention has been illustrated and described as embodied in the context of a piston compressor, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

[0055] Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

[0056] What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

[0057] List of Reference Characters

[0058]1 piston compressor

[0059]2 electric motor

[0060]3 mechanical drive

[0061]4 compressor casing

[0062]5 inner chamber

[0063]6 low-pressure cover

[0064]7 high-pressure cover

[0065]8 low-pressure chamber

[0066]9 low-pressure piston

[0067]10 high-pressure chamber

[0068]11 high-pressure piston

[0069]12 common piston rod

[0070]13 entrance

[0071]14 inlet check valve

[0072]15 outlet

[0073]16 outlet check valve

[0074]17 bypass channel

[0075]18 overflow check valve

[0076]19 free chamber

[0077]20 first casing opening

[0078]21 second casing opening

[0079]22 third casing opening

[0080]23 suction chamber

[0081]24 inlet opening

[0082]25 connection channel

[0083]26 suction bore hole

[0084]27 admission check valve

[0085]28 exhaust check valve

[0086]29 admission check valve of free chamber common wall

[0087]30 mechanical drive casing

[0088]31 motor casing

[0089]32 common wall of free chamber and mechanical drive casing

[0090]33 common wall of mechanical drive casing and motor casing

[0091]34 exhaust check valve of free chamber common wall

[0092]35 exhaust check valve of mechanical drive common wall 

1. A multistage piston compressor, comprising a piston compressor (1), an electrical motor (2) and a mechanical drive (3), which mechanical drive (3) converts the rotating motion of the electric motor (2) into an oscillating linear motion and transfers the oscillating linear motion onto the piston compressor (1), wherein a multistage piston compressor (1) has at least one volume changeable low-pressure chamber (8) with a low-pressure piston (9) and at least one volume changeable high-pressure chamber (10) with a high-pressure piston (11) and wherein the low-pressure chamber (8) and the high-pressure chamber (10) of the multistage piston compressor (1) are connected by a bypass channel, (17) and wherein the low-pressure piston (9) and the high-pressure piston (11) are formed as a single piece and wherein a volume changeable free chamber (19) is disposed between the low-pressure piston (9) and the high-pressure piston (11), wherein the volume changeable free chamber (19) of the piston compressor (1) is furnished as a pressure and suction chamber for a separate cooling air stream and wherein the volume changeable free chamber (19) is connected to the atmosphere through the inner chamber of the electric motor (2).
 2. The single stage and multistage piston compressor according to claim 1, wherein the volume changeable free chamber (19) has a connection to the outer atmosphere through an admission check valve (27).
 3. The single stage or multistage piston compressor according to claim 1, wherein the low-pressure chamber (8) has a suction chamber (23) predisposed and wherein the suction chamber (23) is connected on the one hand to the low-pressure chamber (8) and on the other hand to the inner chamber of the electrical motor (2) and to the outer atmosphere.
 4. The single stage or multistage piston compressor according to claim 3, wherein the inlet opening (24) of the suction chamber (23) or the third casing opening (22) of the electrical motor (2) are disposed outside of the dirt and water endangered region for safeguarding the kind of protection of the electrical motor (2).
 5. A method for cooling of an electrical motor (2) for a multistage piston compressor (1), wherein the air suctioned and ejected by a volume changeable free chamber (19) of the piston compressor (1) is employed as a cooling agent for the electric motor (2), wherein the volume changeable free chamber (19) is disposed between two neighboring pressure pistons of different size.
 6. The method according to claim 5, wherein the air moved by the piston compressor (1) is alternatingly suctioned in from the atmosphere through the inner chamber of the electric motor (2) and removed into the atmosphere.
 7. The method according to claim 5, wherein the air moved by the piston compressor (1) is separately suctioned in from the atmosphere and removed into the atmosphere through the inner chamber of the electrical motor (2).
 8. The method according to claim 6, wherein the air moved by the piston compressor (1) is led simultaneously into the low-pressure chamber (8) of the compressor (1) through the inner chamber of the electric motor (2).
 9. A piston compressor apparatus comprising a movable low pressure piston forming a volume changeable low-pressure chamber (8); a movable high-pressure piston (11) forming volume changeable high-pressure chamber (10); a common piston rod (12) connecting the low-pressure piston to the high pressure piston; a bypass channel (17) disposed in the common piston rod and connecting the low-pressure chamber (8) and the high pressure chamber (10) and wherein the low-pressure piston (9) and the high-pressure piston (11) are formed as a single piece; a volume changeable free chamber (19) disposed between the low-pressure piston (9) and the high-pressure piston (11), wherein the volume changeable free chamber (19) changes volume together with a motion of the low-pressure piston and of the high pressure piston, and wherein the movable low pressure piston (9), the movable high-pressure piston (11), the bypass channel (17) and the volume changeable free chamber (19) form a piston compressor (1); an electrical motor (2) for providing a rotary motion; a motor casing surrounding the electrical motor, wherein the volume changeable free chamber (19) is connected to the atmosphere through the motor casing (31) surrounding the electric motor (2); a mechanical drive (3) connected to the electrical motor and to the piston compressor (1) and for converting the rotating motion of the electric motor (2) into an oscillating linear motion and for transferring the oscillating linear motion onto the piston compressor (1).
 10. The piston compressor according to claim 9 further comprising an admission check valve (27) associated with the volume changeable free chamber (19), wherein the volume changeable free chamber (19) has a connection to the outer atmosphere through the admission check valve (27).
 11. The piston compressor according to claim 9 further comprising a suction chamber (23), wherein the low-pressure chamber (8) has the suction chamber (23) predisposed and wherein the suction chamber (23) is connected on the one hand to the low-pressure chamber (8) and on the other hand to the interior of the motor casing (31) and to the outer atmosphere.
 12. The piston compressor according to claim 11 further comprising an inlet opening (24) disposed at the suction chamber (23) and disposed outside of the dirt and water endangered region for safeguarding the kind of protection of the electrical motor (2).
 13. The piston compressor according to claim 11 further comprising a third casing opening (22) disposed at the motor casing (31) and disposed outside of the dirt and water endangered region for safeguarding the kind of protection of the electrical motor (2).
 14. A method for cooling of an electrical motor (2) for a piston compressor (1) comprising employing a low pressure piston having a first size; employing a high pressure piston having a second size different from the first size; solidly connecting thee low-pressure piston to the high pressure piston with a common piston rod (12) forming a bypass channel (17); surrounding the low-pressure piston, the high pressure piston, and the common piston rod with an inner chamber (5) for forming a volume changeable free chamber (19) between the low pressure piston and the high pressure piston; suctioning air into the volume changeable free chamber during an increase in volume of the volume changeable free chamber, wherein the low pressure piston, the high pressure piston, the common piston rod (12) and the inner chamber (5) form a piston compressor (1); ejecting air from the free chamber during a decrease in volume of the volume changeable free chamber, wherein either the air suctioned or the air ejected by the volume changeable free chamber (19) is employed as a cooling agent for an electric motor (2).
 15. The method according to claim 14 further comprising moving air by the piston compressor (1); alternatingly suctioned the air in from the atmosphere through the inner chamber of the electric motor (2) and removing the air into the atmosphere.
 16. The method according to claim 14 further comprising moving the air moved by the piston compressor (1); separately suctioning air in from the atmosphere and removing air into the atmosphere through an inner chamber of a motor casing (31).
 17. The method according to claim 16 further comprising moving the air by the piston compressor (1); leading simultaneously air into the low-pressure chamber (8) of the compressor (1) through the inner chamber of the motor casing (31).
 18. A piston compressor apparatus comprising a movable low pressure piston forming a volume changeable low-pressure chamber (8); a movable high-pressure piston (11) forming volume changeable high-pressure chamber (10); a bypass channel (17) connecting the low-pressure chamber (8) and the high pressure chamber (10) and wherein the low-pressure piston (9) and the high-pressure piston (11) are formed as a single piece; a volume changeable free chamber (19) disposed between the low-pressure piston (9) and the high-pressure piston (11), wherein the volume changeable free chamber (19) changes volume together with a motion of the low-pressure piston and of the high pressure piston, and wherein the movable low pressure piston (9), the movable high-pressure piston (11), the bypass channel (17) and the volume changeable free chamber (19) form a piston compressor (1); an electrical motor (2) providing a rotary motion; a motor casing surrounding the electrical motor, wherein the volume changeable free chamber (19) is connected to the atmosphere through the motor casing (31) surrounding the electric motor (2); a mechanical drive (3) connected to the electrical motor and to the piston compressor (1) and for converting the rotating motion of the electric motor (2) into an oscillating linear motion and for transfering the oscillating linear motion onto the piston compressor (1).
 19. The piston compressor according to claim 18 further comprising a check valve associated with the volume changeable free chamber (19), wherein the volume changeable free chamber (19) has a connection to the outer atmosphere through the check valve.
 20. The piston compressor according to claim 19 further comprising an additional check valve associated with the volume changeable free chamber (19), wherein the additional check valve is associated with the volume changeable free chamber and controls air flow through the motor casing, wherein the additional check valve is of an admission type when the check valve is of an exhaust type and wherein the additional check valve is of an exhaust type when the check valve is of an admission type. 